Hitherto, a liquid crystal element in which a liquid crystal and a transparent polymer resin are compounded to fix a liquid crystal alignment is called a polymer-stabilized liquid crystal (PSLC), has attracted attention since the element exhibit excellent optical properties and response properties, and has been used in displays and the like. As the liquid crystal element, a liquid crystal composition and an element suitable for an AM (active matrix) element have been disclosed (Patent Document 1).
Among liquid crystal elements, a transmissive-scattering liquid crystal element that is obtained by compounding a liquid crystal and a transparent polymer resin and utilizes the difference in refractive index between the polymer and the liquid crystal or between liquid crystal domains exhibits a high efficiency of utilization of visible light since it does not require a polarizing plate, and hence the element has attracted attention. As the transmissive-scattering electro-optical element, in addition to PSLC, polymer dispersed liquid crystals (PDLC) have been widely known. In the former, a minute amount of polymer ranges as a reticulate network in a continuously spread liquid crystal phase and the latter has a structure in which liquid drops of a liquid crystal phase are dispersed in a polymer film (Non-Patent Document 1).
These liquid crystal elements have been in practical use as light modulating shutters in windows, doors, partitions, and the like of vehicles such as electric trains and automobiles, buildings such as office buildings and hospitals, for the purpose of design, protection of privacy, and the like. Moreover, they have been used as display devices for displaying characters and diagrams.
In such devices, in general, since the use time in a transparent state is overwhelmingly long, it is desired to have a reverse mode electro-optical effect that acts so that the element is transparent at the time when voltage is not applied and becomes in a scattered state at the time when voltage is applied, from the viewpoint of electric power saving.
In order to realize the reverse mode transmissive-scattering electro-optical element, generally, there has been known a method of photocuring a photocurable liquid crystal composition with retaining a transparent state such as homogeneous, planer, or homeotropic one to form a composite of a liquid crystal and a polymer resin. As such a liquid crystal element, there have been known PSCT (Polymer Stabilized Cholesteric Texture) utilizing transmission and scattering of visible light by phase change of a chiral nematic liquid crystal having a positive dielectric constant anisotropy as PSLC and a method of using a liquid crystal having a negative dielectric constant anisotropy as PDLC. However, the latter has problems that temperature dependency of visible light transmittance is large, view angle dependency is large, the liquid crystal is expensive, or the like. On the other hand, the former has advantages that temperature dependency of the visible light transmittance is small, view angle dependency of the visible light transmittance is small, and the response rate of the element is fast, and hence is considered promising.
As a process for producing the PSCT-method liquid crystal element, there is opened to public in Patent Document 2 a method of inducing a phase separation into a chiral nematic liquid crystal phase and a polymer resin phase by photopolymerization to form a reticulate polymer network, in a liquid crystal composition in which a minute amount of a polymerizable monomer and a polymerization initiator are mutually dissolved in a chiral nematic liquid crystal having a positive dielectric constant anisotropy.
As a characteristic feature of the polymerizable monomer for use in the PSCT method, in general, it has been known that the monomer has (i) crosslinking ability, (ii) linearity, and (iii) a rigid core (Non-Patent Document 2). As the polymerizable monomer, in Patent Document 3, a curable compound having a chroman structure as a partial structure is mentioned and also a liquid crystal element composed of a resin derived from the curable compound and a chiral nematic liquid crystal and having a good optical properties is mentioned.
In Patent Document 4, a composite containing a cured product of a curable compound having a 4,4′-biphenylene skeleton wherein a curing rate is improved and a liquid crystal is described. Moreover, Patent Document 5 describes that a polymerized product of a 4,4′-bisacryloyloxybiphenyl compound is used as a transparent solid substance of a light adjusting layer, and the element is produced for a short period of time by ultraviolet irradiation at a high illuminance.